Loran-C is a navigation system from which hyperbolic lines of position are determined by measuring the difference in times of arrival of pulses from widely spaced, synchronized transmitting stations. All Loran-C transmitters operate at a fixed frequency of 100 KHz (kilohertz), and 99% of the radiated energy is contained within the band of 90-110 KHz. The Loran-C system achieves high accuracy of time difference measurement by phase comparison of the RF (radio frequency) carrier within the pulse transmissions. RF cycle identification is accomplished by measurement of the pulse envelope.
Loran-C transmitters achieve high stability by timing transmissions using cesium time standards. High accuracy is achieved by an initial grid calibration. This accuracy is maintained through the use of system control monitors which detect small system errors due to changes in velocity of propagation, clock drift, etc. and command small corrections to maintain full system time difference repeatability of the order of 20 nanoseconds throughout the service area.
In the prior art, the system control monitors are always at least ten miles distant from a transmitting station because measurement of time difference or time of arrival at transmitting sites has been suspect because:
(a) the local signal pickup is from the near field and the distant signal pickup is from the far field;
(b) there might be reradiation from the transmitting antenna into the receiving antenna; and
(c) there is a huge signal unbalance and the probability exists of extraneous pickup affecting the phase accuracy.